Photoacoustic remote sensing (pars), and related methods of use

ABSTRACT

A photoacoustic remote sensing system (PARS) for imaging a subsurface structure in a sample, comprising one or more laser sources configured to generate a plurality of excitation beams configured to generate pressure signals in the sample at an excitation location, and a plurality of interrogation beams incident on the sample at the excitation location, a portion of the plurality of interrogation beams returning from the sample that is indicative of the generated pressure signals, an optical system configured to focus the plurality of excitation beams at a first focal point and the plurality of interrogation beams at a second focal point, the first and second focal points being below the surface of the sample, and a plurality of detectors each configured to detect a returning portion of at least one of the plurality of interrogation beams.

FIELD

This application relates to the field of biomedical optics imaging and,in particular, to a laser and ultrasound-based method and system for invivo or ex vivo, non-contact imaging of biological tissue.

BACKGROUND

Photoacoustic imaging is an emerging hybrid imaging technology providingoptical contrast with high spatial resolution. Nanosecond or picosecondlaser pulses fired into tissue launch thermo-elastic-induced acousticwaves which are detected and reconstructed to form high-resolutionimages. Photoacoustic imaging has been developed into multipleembodiments, including photoacoustic tomography (PAT), photoacousticmicroscopy (PAM), optical-resolution photoacoustic microscopy (OR-PAM),and array-based PA imaging (array-PAI). In photoacoustic tomography(PAT), signals are collected from multiple transducer locations andreconstructed to form a tomographic image in a way similar to X-ray CT.In PAM, typically, a single element focused high-frequency ultrasoundtransducer is used to collect photoacoustic signals. A photoacousticsignal as a function of time (depth) is recorded for each position in amechanically scanned trajectory to form a 3-D photoacoustic image. Themaximum amplitude as a function of depth can be determined at each x-yscan position to form a maximum amplitude projection (MAP) C-scan image.Photoacoustic microscopy has shown significant potential for imagingvascular structures from macro-vessels all the way down tomicro-vessels. It has also shown great promise for functional andmolecular imaging, including imaging of nanoparticle contrast agents andimaging of gene expression. Multi-wavelength photoacoustic imaging hasbeen used for imaging of blood oxygen saturation, by using known oxy-and deoxy-hemoglobin molar extinction spectra.

In traditional photoacoustic imaging, spatial resolution is due toultrasonic focusing and can provide a depth-to-resolution ratio greaterthan 100. In OR-PAM, penetration depth is limited to −1 mm in tissue(due to fundamental limitations of light transport) but resolution ismicron-scale due to optical focusing. OR-PAM can provide micron-scaleimages of optical absorption in reflection-mode, in vivo, something thatno other technique can provide. OR-PAM is capable of imaging bloodvessels down to capillary size noninvasively. Capillaries are thesmallest vessels in the body and much crucial biology occurs at thislevel, including oxygen and nutrient transport. Much can go wrong at thecapillary level too. In cancers, cells have an insatiable appetite foroxygen and nutrients to support their uncontrolled growth. They invoke arange of signaling pathways to spawn new vessels in a process known asangiogenesis and these vessels typically form abnormally. Tumors areoften highly heterogeneous and have regions of hypoxia. Photoacousticimaging has demonstrated the ability to image blood oxygen saturation(SO2) and tumor hypoxia in vivo.

In most photoacoustic and ultrasound imaging systems, piezoelectrictransducers have been employed, in which an ultrasound coupling mediumsuch as water or ultrasound gel is required. However, for many clinicalapplications such as wound healing, burn diagnostics, surgery, and manyendoscopic procedures, physical contact, coupling, or immersion isundesirable or impractical.

The detection of ultrasound in photoacoustic imaging has, untilrecently, relied on ultrasonic transducers in contact with thebiological tissue or an ultrasonic coupling agent both of which havemajor drawbacks as described above. Some detection strategies to solvingthe non-contact optical interferometric sensing problems associated withphotoacoustic imaging have been reported.

Optical means of detecting ultrasound and photoacoustic signals havebeen investigated over a number of years; however, to date, no techniquehas demonstrated practical non-contact in vivo microscopy in reflectionmode with confocal resolution and optical absorption as the contrastmechanism.

Most previous approaches detected surface oscillations withinterferometric methods. Others used interferometry to observephotoacoustic stresses, including optical coherence tomography (OCT)methods. These methods offer potential sensitivity to the scatteredprobe beam phase modulations associated with motion of scatterers,subsurface and surface oscillations, as well as unwanted vibrations.They are also sensitive to complex amplitude reflectivity modulations.

One example of a low-coherence interferometry method for sensingphotoacoustic signals was proposed in U.S. pregrant publication no.2014/0185055 to be combined with an optical coherence tomography (OCT)system, resulting in 30 μm lateral resolution.

Another prior art system is described in U.S. pregrant publication no.2012/0200845 entitled “Biological Tissue Inspection Method and System”,which describes a noncontact photoacoustic imaging system for in vivo orex vivo, non-contact imaging of biological tissue without the need for acoupling agent.

Other systems use a fiber based interferometer with opticalamplification to detect photoacoustic signals and form photoacousticimages of phantoms with acoustic (not optical) resolution. However,these systems suffer from a poor signal-to-noise ratio, whereas othercontact-based photoacoustic systems offer significantly improveddetection capabilities. Furthermore, in vivo imaging was notdemonstrated, and optical-resolution excitation was not demonstrated.

Industrial laser ultrasonics has used interferometry to detect acousticsignatures due to optical excitation of inanimate objects fornon-destructive testing. This approach has been adapted to detectultrasound ex vivo in chicken breast and calf brain specimens, however,optical-resolution focusing of the excitation light was not examined.

Laser Doppler vibrometry has been a powerful non-contact vibrationsensing methodology, however, weak signal-to-noise and poor imagequality have proven to be a limitation when sensing deep-tissue signalsfrom broad-beam photoacoustic excitation.

Similarly, Mach Zehnder interferometry and two-wave mixinginterferometry have been used previously for sensing photoacousticsignals. However, many such techniques still require direct contact orfluid coupling; they have not offered in vivo studies or opticalresolution for phantom studies.

The photoacoustic remote sensing (PARS) (including thenon-interferometric photoacoustic remote sensing (NI-PARS)) systemsdescribed herein are fundamentally different from other approaches fordetection ultrasound/photoacoustic signals. The PARS takes advantage ofa excitation beam co-focused and co-scanned with an interrogation beam.Specifically, the PARS uses nJ-scale pulse energies focused to neardiffraction-limited spots, and not the conventional broad excitationbeams delivered over broad areas. Furthermore, in the NI-PARS, thedetection mechanism is based on a non-interferometric sensing. Ratherthan detecting surface oscillations, pressure-induced refractive-indexmodulation resulting from initial pressure fronts can be sampled rightat their subsurface origin where acoustic pressures are large. Thenon-interferometric nature of detection along with the short-coherencelengths of the interrogation laser preclude detection of surface andsub-surface oscillations to provide only the initial pressure signals.

SUMMARY

According to an example, a photoacoustic remote sensing system (PARS)for imaging a subsurface structure in a sample may comprise one or morelaser sources configured to generate a plurality of excitation beamsconfigured to generate pressure signals in the sample at an excitationlocation, and a plurality of interrogation beams incident on the sampleat the excitation location, a portion of the plurality of interrogationbeams returning from the sample that is indicative of the generatedpressure signals. The PARS may further comprise an optical systemconfigured to focus the plurality of excitation beams at a first focalpoint and the plurality of interrogation beams at a second focal point,the first and second focal points being below the surface of the sample,and a plurality of detectors each configured to detect a returningportion of at least one of the plurality of interrogation beams. The oneor more laser sources may be a plurality of laser sources. Each of theplurality of excitation beams may have a different wavelength. Theplurality of excitation beams may include a near-infrared beam, ashort-wave infrared beam, a UVC beam, a UVB beam, a UVA beam, andvisible light. The plurality of excitation beams may be configured to bedelivered sequentially onto the sample, or the plurality of excitationbeams may be configured to be delivered simultaneously onto the sample.The first and second focal points may be at a depth below the surface ofthe sample that is less than 11 μm.

In another example, a photoacoustic remote sensing system (PARS) forimaging a subsurface structure in a sample may comprise one or morelaser sources configured to generate at least one excitation beamconfigured to generate ultrasonic signals in the sample at an excitationlocation, wherein the at least one excitation beam is directed to thesample along a first path, and at least one interrogation beam incidenton the sample at the excitation location and directed to the samplealong a second path that is offset from the first path, at least oneportion of the at least one interrogation beam returning from the samplethat is indicative of the generated ultrasonic signals, wherein thereturning portion of the at least one interrogation beam returns along athird path that is offset from each of the first path and the secondpath. The PARS may further include a first optical system configured tofocus the at least one excitation beam at a first focal point, a secondoptical system configured to focus the at least one interrogation beamat a second focal point, the first and second focal points being belowthe surface of the sample, and at least one detector configured todetect at least one returning portion of the at least one interrogationbeam. The angle between the first path and second path may besubstantially similar to an angle between the second path and the thirdpath. The angle between the first path and the third path may besubstantially similar to an angle between the first path and the thirdpath.

In another example, a photoacoustic remote sensing system (PARS) forimaging a subsurface structure in a sample may comprise one or morelaser sources configured to generate at least one excitation beamconfigured to generate ultrasonic signals in the sample at an excitationlocation, and at least one interrogation beam incident on the sample atthe excitation location, at least one portion of the at least oneinterrogation beam returning from the sample that is indicative of thegenerated ultrasonic signals. The PARS may further comprise an opticalsystem configured to focus the at least one excitation beam at a firstfocal point and the at least one interrogation beam at a second focalpoint, the first and second focal points being below the surface of thesample, and a polarizing modulation detector configured to detect apolarization modulation of the at least one returning portion.

According to another example, a photoacoustic remote sensing system(PARS) for imaging a subsurface structure in a sample may comprise oneor more laser sources configured to generate at least one excitationbeam configured to generate ultrasonic signals in the sample at anexcitation location, and at least one interrogation beam incident on thesample at the excitation location, at least one portion of the at leastone interrogation beam returning from the sample that is indicative ofthe generated ultrasonic signals. The PARS may further comprise anoptical system configured to focus the at least one excitation beam at afirst focal point and the at least one interrogation beam at a secondfocal point, the first and second focal points being below the surfaceof the sample, and a phase modulation detector configured to detect aphase modulation of the at least one returning portion.

In another example, a photoacoustic remote sensing system (PARS) forimaging a structure in a sample may comprise one or more laser sourcesconfigured to generate at least one excitation beam configured togenerate pressure in the sample at an excitation location, wherein theone or more laser sources also are configured to generate at least oneinterrogation beam incident on the sample at the excitation location, atleast one portion of the at least one interrogation beam returning fromthe sample that is indicative of the generated ultrasonic/pressuresignals, and a detector configured to detect at least one light propertyof the at least one returning portion. The at least one light propertymay include polarization, phase, amplitude, scattering,auto-fluorescence, and second harmonic generation. The at least onelight property may include a plurality of light properties, and thedetector may be configured to detect the plurality of light propertiessimultaneously or separately. The PARS may be configured to image thestructure of the sample through a glass window holding the sample. ThePARS may comprise a plurality of laser sources configured to generate aplurality of excitation beams simultaneously, a plurality ofinterrogation beams simultaneously, or at least one excitation beam andat least one interrogation beam simultaneously. The PARS may include anendoscope. Furthermore, the PARS may further include an optical systemconfigured to focus the at least one excitation beam at a first focalpoint and the at least one interrogation beam at a second focal point,wherein the PARS is configured to scan the optical system while thesample remains stationary.

The above-mentioned PARS examples may be used in one or more of thefollowing applications: imaging histological samples; imaging cellnuclei; imaging proteins; imaging cytochromes; imaging DNA; imaging RNA;imaging lipids; imaging of blood oxygen saturation; imaging of tumorhypoxia; imaging of wound healing, burn diagnostics, or surgery; imagingof microcirculation; blood oxygenation parameter imaging; estimatingblood flow in vessels flowing into and out of a region of tissue;imaging of molecularly-specific targets; imaging angiogenesis forpre-clinical tumor models; clinical imaging of micro- andmacro-circulation and pigmented cells; imaging of the eye; augmenting orreplacing fluorescein angiography; imaging dermatological lesions;imaging melanoma; imaging basal cell carcinoma; imaging hemangioma;imaging psoriasis; imaging eczema; imaging dermatitis; imaging Mohssurgery; imaging to verify tumor margin resections; imaging peripheralvascular disease; imaging diabetic and/or pressure ulcers; burn imaging;plastic surgery; microsurgery; imaging of circulating tumor cells;imaging melanoma cells; imaging lymph node angiogenesis; imagingresponse to photodynamic therapies; imaging response to photodynamictherapies having vascular ablative mechanisms; imaging response tochemotherapeutics; imaging response to anti-angiogenic drugs; imagingresponse to radiotherapy; estimating oxygen saturation usingmulti-wavelength photoacoustic excitation; estimating venous oxygensaturation where pulse oximetry cannot be used; estimating cerebrovenousoxygen saturation and/or central venous oxygen saturation; estimatingoxygen flux and/or oxygen consumption; imaging vascular beds and depthof invasion in Barrett's esophagus and/or colorectal cancers; functionalimaging during brain surgery; assessment of internal bleeding and/orcauterization verification; imaging perfusion sufficiency of organsand/or organ transplants; imaging angiogenesis around islet transplants;imaging of skin-grafts; imaging of tissue scaffolds and/or biomaterialsto evaluate vascularization and/or immune rejection; imaging to aidmicrosurgery; guidance to avoid cutting blood vessels and/or nerves;imaging of contrast agents in clinical or pre-clinical applications;identification of sentinel lymph nodes; non- or minimally-invasiveidentification of tumors in lymph nodes; imaging of genetically-encodedreporters, wherein the genetically-encoded reporters include tyrosinase,chromoproteins, and/or fluorescent proteins for pre-clinical or clinicalmolecular imaging applications; imaging actively or passively targetedoptically absorbing nanoparticles for molecular imaging; imaging ofblood clots; or staging an age of blood clots.

The various embodiments described above are not limited to a particularphotoacoustic remote sensing (PARS) system. Rather, they may be appliedto the various PARS systems described herein and in U.S. Pat. Nos.10,117,583 B2, 10,327,646 B2, U.S. Patent Publication No. 2019/0104944A1, U.S. Patent Publication No. 2019/0320908 A1, U.S. Patent PublicationNo. 2018/0275046 A1, and International PCT Publication No.WO2019/145764, all of which are incorporated by reference herein intheir entireties.

Other aspects will be apparent from the description and claims below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features will become more apparent from the followingdescription in which reference is made to the appended drawings, thedrawings are for the purpose of illustration only and are not intendedto be in any way limiting, wherein:

FIGS. 1A-1C are block diagrams of a photoacoustic remote sensing (PARS)microscopy system, according to various embodiments.

FIG. 2A is a block diagram of a PARS, according to other embodiments.

FIGS. 2B and 2C are illustrations of excitation and detection beams on asample.

FIG. 2D is a three-dimensional illustration showing excitation anddetection beams applied to a sample, along with a returning portion ofthe detection beam.

FIGS. 3A-3B are block diagrams of a PARS, according to otherembodiments.

FIG. 4 is a block diagram of a PARS, according to another embodiment.

FIGS. 5A-5I are representative drawings of different overlaps betweenthe excitation and interrogation beams on a sample.

FIGS. 6A-6C are block diagrams of sensing systems in an endoscopyconfiguration, according to various embodiments.

FIG. 7 is a block diagram of a sensing system integrated with anotheroptical imaging system.

DESCRIPTION

Reference will now be made in detail to examples of the presentdisclosure, which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts. In the discussion thatfollows, relative terms such as “about,” “substantially,”“approximately,” etc. are used to indicate a possible variation of ±10%in a stated numeric value.

Photoacoustic imaging is a biomedical imaging modality that uses laserlight to excite tissues. Energy absorbed by chromophores, or any otherabsorber, is converted to acoustic waves due to thermo-elasticexpansion. These acoustic signals are detected and reconstructed to formimages with optical absorption contrast. Photoacoustic imaging (PA) hasbeen shown to provide exquisite images of microvessels and is capable ofimaging blood oxygen saturation, gene expression, and contrast agents,among other uses. In most PA and ultrasound imaging systems,piezoelectric transducers have been employed, in which an ultrasoundcoupling medium such as water or ultrasound gel is required. However,for many clinical applications such as wound healing, burn diagnostics,surgery, and many endoscopic procedures, physical contact, coupling, orimmersion is undesirable or impractical. The systems described hereinare capable of in vivo optical-resolution photoacoustic microscopy usingnon-contact non-interferometric sensing without use of any ultrasoundmedium.

The systems described herein, i.e., photoacoustic remote sensing (PARS)microscopy systems, are based on the idea of focusing excitation lightto an excitation spot, e.g., an aperture-limited diffraction-limitedspot, which is larger than the absolute diffraction-limited spot, anddetecting photoacoustic signals using a confocal interrogation beamco-focused with the excitation spot. While previous approaches use abroad excitation beam with powerful lasers delivering mJ-J of pulseenergy over a broad area, the PARS microscopy technique described hereinuses nJ- or pico-joules scale pulse energies focused to excitationspots, e.g., near diffraction-limited spots. It is noted that largerpulse energies may be delivered depending on the size of the excitationspots. Excitation spot sizes, i.e., the diameter of the spots, are notparticularly limited. In some examples, excitation spot sizes may beless than 30 μm, less than 20 μm, less than 10 μm, or less than 1 μm.Larger pulse energies may also be appropriate in instances in which theexcitation is significantly larger than the diffraction limit. Whenfocusing into tissue, the surface fluence can be maintained belowpresent ANSI limits for laser exposure but the ballistically-focusedlight beneath the tissue can create fluences transiently far above theANSI limits (as is done in other microscopy methods). In PARS, thismeans that very large local fluences ˜J/cm2 are created within amicron-scale spot, generating large initial acoustic pressures. Forexample, at 532-nm excitation wavelength, imaging a capillary with 500mJ/cm² local fluence would result in an initial pressure on the order of100 MPa locally. In the PARS approach, large optically-focusedphotoacoustic signals are detected as close to the photoacoustic sourceas possible, which is done optically by co-focusing an interrogationbeam with the excitation spot.

Some examples of interferometric PARS systems, e.g., coherence gatedphotoacoustic remote sensing (CG-PARS) systems, may perform opticaldepth scanning of samples. CG-PARS and other PARS systems may beoptimized in order to take advantage of a multi-focus design forimproving the depth-of-focus of 2D and 3D optical resolution (OR) PARSimaging. The chromatic aberration in a collimating and objective lenspair may be harnessed to refocus light from a fiber into the object sothat each wavelength is focused at a slightly different depth location.Using these wavelengths simultaneously may be used to improve the depthof field and signal to noise ratio (SNR) of PARS images. During PARSimaging, depth scanning by wavelength tuning may be performed.

Other examples of PARS systems may not perform optical depth scanning.Since depth scanning is not performed with certain embodiments ofNI-PARS, NI-PARS can perform in near real time using a high pulserepetition laser and fast scanning mirrors. However, most previousnon-contact photoacoustic detection methods have not shown real-timeimaging capability and optical resolution was not demonstrated.Embodiments of the disclosure optically focus a pulsed excitation laserinto superficial tissues to generate high micro-scale initial pressures.Then, these large optically-focused photoacoustic signals are harvestedas close to the photoacoustic source as possible. This is done bydetecting photoacoustic signals using a confocal interrogation beamco-focused and co-scanned with the excitation spot. Local initialpressures are very large when optical focusing and thermal confinementconditions are applied. These large initial pressures can causesignificant refractive index mismatch regions which are measured by theNI-PARS as changes in reflected light.

Furthermore, PARS is not limited to the application of a singularexcitation beam and/or a singular detection/interrogation beam. Forexample, a PARS may focus a plurality of excitation beams to a spot,e.g., aperture-limited diffraction-limited spot, neardiffraction-limited spot, and/or a plurality of interrogation beams atthe excitation spot. As discussed above, the size of the excitation spotis not particularly limited, and may be less than 30 μm, less than 20μm, less than 10 μm, or less than 1 μm. PARS may further include aplurality of detectors configured to detect the returning photoacousticsignals. Such systems may provide additional advantages and benefits,including flexibility and sequential sample interrogation.

Embodiments of the present disclosure are related to anultrasound/photoacoustic imaging detection mechanism based onpressure-induced refractive-index modulation as well as real-timenon-contact detection. This approach contemplates interrogatingsubsurface absorption with optical resolution using a non-contactsystem. The range of subsurface depth is not particularly limited, andin some examples, may range from about 50 nm to 8 mm. Thus, subsurfaceabsorption depths in some examples may be very small such as in, e.g.,skin samples, or histology glass slides. In such instances, some portion(e.g., half) of the excitation spot may be inside the sample whileanother portion (e.g., the other half) may be outside of the sample.

The high sensitivity and the fine resolution of the proposed systemoffer performance comparable to other in vivo optical resolutionphotoacoustic microscopy systems, but in a non-contact reflection modesuitable for many clinical and pre-clinical applications.

Various embodiments of photoacoustic remote sensing microscopy systems(PARS) are depicted through FIGS. 1A-4 . Variations to the depictedsystems will be apparent to those skilled in the art.

Referring to FIG. 1A, a block diagram of an embodiment of a PARS 10 a. Amulti-wavelength fiber excitation laser 12 is used in multi focus formto generate photoacoustic signals. Excitation laser 12 may operate inthe visible, ultraviolet or near-infrared spectrum, although theparticular wavelength may be selected according to the requirements ofthe particular application. An excitation beam 17 passes through amulti-wavelength unit 40, and both excitation beam 17 and aninterrogation beam 16 pass through a lens system 42 to adjust theirfocus on a sample 18. Excitation beam 17 and interrogation beam 16, thepaths of which are diametrically across from one another, will becombined using a beam combiner 30. The acoustic signatures areinterrogated using either a short or long-coherence length probe beam 16from a detection laser 14 that is co-focused and co-aligned with theexcitation spots on sample 18. Interrogation/probe beam 16 passesthrough a polarizing beam splitter 44 and quarter wave plate 56 to guidethe reflected light 20 from sample 18 to the photodiode 46. However,PARS 10 a is not limited to including polarizing beam splitter 44 andquarter wave plate 56. The aforementioned components may be substitutedfor fiber-based, equivalent components, e.g., a circulator, coupler,WDM, and/or double-clad fiber, that are non-reciprocal elements. Suchelements may receive light from a first path, but then redirect saidlight to a second path. A combined beam 21 of excitation beam 17 andinterrogation beam 16 will be scanned by scanning unit 19. The scannedcombined beam 21 will pass through an objective lens 58 and focus on thesample 18. The reflected beam 20 returns along the same path and isanalyzed by detection unit 22. Unit 22 includes amplifier 48, fast dataacquisition card 50 and computer 52.

FIG. 1B shows another embodiment of a PARS 10 b, in which scanning unit19 (shown in FIG. 1A) is replaced by scanning unit 11 in order to scan(move) the sample 18 in relation to the fixed combined beams 21. In someother embodiments, PARS systems may include both scanning unit 11 andscanning unit 19, thereby having scanning units on opposite ends ofcombined beam 21.

FIG. 1C is another block diagram of an embodiment of a PARS 10 c. PARS10 c includes three excitation lasers 12 a-12 c configured to providethree excitation beams 17 a-17 c, three detection lasers 14 a-14 cconfigured to provide three interrogation beams 16 a-16 c, and threedetection units 22 a-22 c to receive and analyze reflected beams 20 a-20c. It is noted, however, that the number of excitation lasers, detectionlasers, and detectors is not particularly limited, and any suitablenumber of lasers and configurations thereof may be used, such as, forexample, two, four, five, or more. Similar to PARS 10 a, excitationbeams 17 a-17 c and interrogation beams 16 a-16 c combine via beamcombiner 30 to focus combined beam 21, passing through objective lens58, onto sample 18. Reflected beams 20 a-20 c reflect in directionsopposite of combined beam 21, and are received by detection units 22a-22 c. Beam combiner 30 may serve additional functions in PARS 10 c,including serving as a polarizing beam splitter of interrogation beams16 a-16 c and as a guide for re-directing reflected beams 20 a-20 ctoward detection units 22 a-22 c. Detection units 22 a-22 c mayrespectively include an amplifier (not shown), a fast data acquisitioncard (not shown), and a computer (not shown), such as amplifier 48, fastdata acquisition card 50, and computer 52 set forth above with respectto FIG. 1A.

PARS 10 c, including a plurality of excitation beams and/orinterrogation beams and or detectors, may provide users with the optionof applying beams of varying properties, e.g., wavelength, for variousaims. For example, to image deep-inside biological tissues, it may bedesirable to use a deeply-penetrating (long transport mean-free-path)optical wavelength such as a short-wave infrared wavelength. An exampleof a deeply-penetrating wavelength is 1310 nm, which is typically usedin PARS for deep imaging. Alternatively, when imaging superficialtargets, there may be geometric benefits (in terms of a smaller focalspot size) and sensitivity benefits (in terms of increased scattering)to using a shorter, visible wavelength, such as 630 nm. The combinationof such geometric and sensitivity benefits can result in several ordersof magnitude difference in the amount of returned light from an imagedsample. For instance, the focal spot area for 500 nm light will beroughly 9 times smaller than that of 1500 nm light for the same focusingoptics. Likewise, for biological tissues, the scattering at 500 nm canbe 3 to 4 times stronger than at 1500 nm, for example. Thus, suchbenefits from using a wavelength of 500 nm, as opposed to a wavelengthof 1500 nm, may ultimately result in a 30 to 40-fold detectionsensitivity improvement at superficial depths. It is noted thatexcitation wavelengths are not particularly limited to theaforementioned example values, and may be any wavelengths suitable forthe intended purpose. The two properties of deep sample penetration andimproved superficial performance may also be desirable for use at thesame time, or as a switchable option depending on the desired outcome ofan imaging session. For example, both beams may be used at the same timeif imaging near-surface capillary vessels followed by deeper vesselswith a single volumetric scan. The superficial structures may benefitfrom the improved resolution and sensitivity of the shorter detectionwavelength, whereas the deeper structures may only be recovered by usingthe infrared wavelengths. However, the use of two beams at the same timemay provide too much exposure to optical radiation, and thus a switchingapproach may be taken where the shorter detection wavelength is tradedfor the longer wavelength detection at an appropriate depth in thesample. Thus, PARS having a plurality of excitation beams and/orinterrogation beams and/or detectors may allow a user to implement twoor more detections in the same system, thereby allowing the user toexamine the effectiveness of each detection on a sample. Some samplesmay provide specific improved contrast for a given detection wavelengthover others, due to the nature of light scattering and extinction atparticular wavelengths. Multiple detection paths may also be combinedusing free-space optical beam combiners such as a dichroic orbeam-splitters or using fiber-based devices such as couplers orwavelength division multiplexers.

A plurality of excitation wavelengths may also be used sequentiallywhile acquiring multiplexed/functional information from a single sample,such as imaging oxy- and deoxyhemoglobin for visualization of bloodoxygenation, or targeting DNA and cytochrome absorption peak to extracthistological information from a tissue sample. To facilitate rapid andconsistent imaging, which may minimize the potential for motionartifacts and may allow for video-rate real-time multiplexed/functionalimaging, the plurality of excitation wavelengths may be used in closesuccession to one another, for example, up to MHz-range repetitionrates, so that the plurality of excitation beam sources are set-up andactive simultaneously in the same-PARS. Multiplexed/functionalinformation may also be extracted from a sample using variations inpulse-widths. These widths are not particularly limited, and may varyfrom the thermal and stress confinement conditions in the hundreds ofnanoseconds, or down to the femtosecond range. For example, oxygenatedand deoxygenated hemoglobin can be separated using two 532 nm sources,one which provides picosecond-scale pulse widths and the other operatingin the nanosecond regime (provides nanosecond-scale pulse widths). Ingeneral, PARS excitation paths may include any combination ofwavelengths, pulse widths, repetition rates, and pulse energies, whichprovide various benefits in terms of sample exposure, imagingsensitivity, imaging specificity, and chromophore de-mixing. Themultiple excitation beam paths may be combined using free-space opticalbeam combiners such as a dichroic or beam-splitters or using fiber-baseddevices such as couplers or wavelength division multiplexers.

Thus, a PARS including a combination of multiple detection/interrogationbeams and excitation beams may provide highly tunable imagingparameters. As discussed above, such a system may be configured to imagedeeply in scattering tissue to target near-infrared blood absorption.The same system may be configured to use a short-wave infrared detectionproviding penetration depths approaching 3 mm for optical resolutionsthat are less than 2 μm, and beyond this depth with decreased resolvingpowers. This may be done sequentially or simultaneously within the samePARS. The same system may also use a UVC excitation, having wavelengthsof 200 to 280 nm, to target DNA absorption, and use UVA detection,having wavelengths of 315 to 400 nm, to provide superficial imagingperformance with resolutions on the order of several hundred nanometers.UVB beams also may be utilized for excitation/detection.

FIG. 2A shows an embodiment of PARS 10 d, which includes individualoptical systems, adjacent to one another, which are separatelyconfigured to focus excitation beam 16, interrogation beam 17, andreceive reflected beam 20, respectively. In PARS 10 d, excitation beam16 and interrogation beam 17 are not combined via a beam combiner, andco-focus on sample 18, via separate focusing optics, i.e., 58 a and 58b. Focusing optics 58 a and 58 b may include any device(s) used toconverge the beam of light, such as an objective lens or curved mirror.It is noted that the central axes of excitation beam 16 andinterrogation beam 17 are angled and offset relative to each other, butthat the angle is not particularly limited. Reflected beam 20 returnsalong a different path that is angled and offset to the axis ofinterrogation beam 16, and reflects back towards focusing optics 58 c,which guides reflected beam 20 to detection unit 22.

Similar to PARS 10 d, FIGS. 2B and 2C further show excitation beam 17and interrogation beam 16 being directed to sample 18 at an anglerelative to one another. However, unlike system 10 d, FIGS. 2B-2Cillustrate the use of refractive optics, as opposed to reflectiveoptics, such as, e.g., mirrors. In FIG. 2B, both excitation beam 17 andinterrogation beam 16 pass through a single objective lens 58, whichco-focuses beams 16 and 17 on sample 18 from two distinct angles. Whilethe refracted portions of beam 16 and 17 are off their respectivelongitudinal axes (of beams 16 and 17 prior to passing through lens 58),the refracted beams are still parallel to said axes such that beams 16and 17 are able to co-focus onto the same spot of sample 18. Becausethere is a single objective lens 58 in FIG. 2B, the angle between beams16 and 17 may be relatively shallow in comparison to systems in whichtwo lenses may be used. However, the use of single objective lens 58 mayalso allow for relatively easier co-alignment of beams 16 and 17 ontosample 18.

In contrast, in FIG. 2C, excitation beam 16 and interrogation beam 17each pass through their respective objective lens, i.e., objective lens58 b and objective lens 58 a. Moreover, beams 16 and 17 remain centeredalong their respective longitudinal axes to co-focus onto the same spotof sample 18. Because there are separate objective lenses 58 b and 58 afor beams 16 and 17, respectively, the range of the angle between beams16 and 17 may be more flexible and larger angles than the embodimentshown in FIG. 2B. Such a configuration may also enable higher levels ofpolarization. However, the embodiment shown in FIG. 2C requiresco-alignment of objective lenses 58 b and 58 a so that beams 16 and 17may co-focus onto sample 18. Other PARS embodiments, may also includeadditional individual optical systems, and/or may be in differentconfigurations or arrangements relative to one another.

The configuration of PARS 10 d, and the beam configurations shown inFIGS. 2A-2C may provide added spatial rejection of undesired randomlyscattered photons, and detect only photons that have been modulated byexcitation laser 12. Since the PARS imaging region is defined by theoverlap of excitation beam 16, detection/interrogation beam 17, andbackwards detection/reflected beam path 20, if these paths are allco-aligned, the interrogated region on sample 18 may be defined by aradial distribution which is commonly shorter than the axialdistribution. This may cause the axial resolution of such imagingsystems to be larger, and thus, worse than the lateral resolution. Byangling excitation beam 16 and interrogation beam 17 relative to eachother, as shown in PARS 10 d and the beams shown in FIGS. 2A-2C, theoverlap may now be defined between the combination of two or threeradial distributions. This allows for the lateral resolution of one ofthe beams to improve upon the axial performance provided by the otherbeam. To maximize this effect, it may be most advantageous to have thethree beams evenly distributed in the azimuth and with around 45 degreeseach to the sample surface. This is shown in FIG. 2C, which illustratessample 18 on a plane, and excitation beam 17, interrogation beam 16, andreflected interrogation beam 20 having beam paths, originating fromsample 18, of congruent azimuth angles 26 a, i.e., 120°. The beam pathsalso have congruent altitude angles 26 b, which may range from 20-90°.However, in other embodiments the altitude angles may vary amongst thebeam paths. Decreasing internal angles between beams 16, 17, and 20 maysimply begin to approach the performance of non-angled PARS fordecreasing internal angles, and become unpractical as angles approach180 degrees since samples are generally flat.

As shown in FIGS. 2A-2C, the angling of the focused paths of excitationbeam 16 and interrogation beam 17 may be achieved through angling of theinput beams into a single focusing element, i.e., objective lens 58shown in FIG. 2B, or by constructing a system with multiple focusingelements which are angled to each other, i.e., objective lens 58 and 15shown in FIG. 2C, or some combination of the two. As a result, the axesof excitation beam 16 and interrogation beam 17 may be angled relativeto one another.

PARS including an excitation source, a detection source, a beam combinercombining excitation beam(s) and interrogation beam(s), focusing optics,and a detector, similar to the embodiment in FIG. 1A, capture intensitymodulations in the collected light/reflected beam from the sample. Thismay be done by sensing the change in scattering from the sample. Othernon-PARS or devices that may perform such a function include scatteringmicroscopes, which may include a detection beam from a detection sourcepassing through a combiner/splitter to focusing optics, which focus thebeam onto a sample, and an intensity detector configured to receivereflected interrogation/detection beams (with no excitation beam).

However, the reflected interrogation beam also contains informationregarding its polarization state and its phase, and there areconventional, non-PARS or devices that may capture polarization andphase accumulation. One such device may be a polarization-basedmicroscope, which is similar to the above described scatteringmicroscope, except a polarization detector is used in place of anintensity detector. Another such device may be a conventional phasemicroscope, which may include a detection beam from a detection sourcepassing through an interferometer to focusing optics, which focus thebeam onto a sample, and a phase detector configured to receive reflectedinterrogation/detection beams that return through the interferometer.Thus, PARS of the present disclosure modulate the scattering propertiesof reflected beam 20 and also respectively modulate the apparentpolarization and phase accumulation within a sample. Such PARS arefurther discussed below, referring to FIGS. 3A and 3B.

FIG. 3A shows another block diagram of an embodiment of PARS 10 g. PARS10 f includes excitation laser 12 configured to provide excitation beam17, and detection laser 14 configured to provide interrogation beam 16.However, as previously discussed, the number of excitation lasers anddetection lasers is not particularly limited, and any suitable number oflasers and configurations thereof may be used. Similar to PARS 10 a,excitation beam 17 and interrogation beam 16 combine via beam combiner30 to focus combined beam 21, passing through objective lens 58, ontosample 18. Furthermore, in this embodiment, beam combiner 30 may alsoserve the function of a polarizing beam splitter of interrogation beam16. However, PARS 10 f does not include the detection unit 22 shown inFIG. 1A. Instead, reflected beam 20 is reflected back through beamcombiner 30, which guides reflected beam 20 to a polarization modulationdetector 23. It is noted that a quarter waveplate is not used in PARS 10g, so that the polarization state of reflected beam 20 may be maintainedwhen guided toward polarization modulation detector 23.

More specifically, to capture polarization modulation, interrogationbeam 16 with a controlled polarization is fed into sample 18, wherereflected light 20 is now separated based on its polarization content.The means by which polarization is controlled in not particularlylimited, and can be, e.g., a conventional polarization controller, andin some embodiments, beam 16 may already be polarized when emitted fromlaser 14. For example, vertically polarized light may be directed to onephotodetector within detector 23 and horizontally polarized light may bedirected to another photodetector within detector 23. Different aspectsof polarization could be used such as linear direction, handedness ofcircular polarized states, and higher-dimensional polarizationdistributions, such as radially and azimuthally polarized states.Separation and characterization of these states may be accomplished withpolarization sensitive detectors, i.e., polarization modulation detector23, quarter wave plate 56, and polarization-sensitive splitters (notshown). This may allow for precise characterization of the polarizationshift, as the modulated value could be directly compared with theun-modulated value at the same sample location.

FIG. 3B shows an embodiment of PARS 10 h also including excitation laser12 configured to provide excitation beam 17, and detection laser 14configured to provide interrogation beam 16. PARS 10 g includes aninterferometer 24 and a phase modulation detector 25. PARS 10 g may bearranged so that interrogation beam 17 passes through interferometer 24and is guided to beam combiner 30, at which interrogation beam 17combines with excitation beam 16. Reflected beam 20 from sample 18returns along the same path of interrogation beam 17 up untilinterferometer 24, at which reflected beam 20 is then guided towards andreceived by phase modulation detector 25.

To capture phase shifting, a phase sensitive detector, i.e., phasemodulation detector 25, is implemented. This may be done with heterodyneand homodyne interferometry, which may capture a component of or thefull quadrature of returning light 20 from sample 18. This would allowfor precise characterization of the phase shift, as the modulated valuecould be directly compared with the un-modulated value at the samesample location.

Any combination of these six light properties (e.g., scattering,polarization, phase, and their respective modulations) may be capturedand analyzed in a PARS via any suitable mechanism, e.g., phasemodulation detector 25 for phase, where the contrast mechanisms mayprovide unique and complementary information. For example, PARS maygenerate strong second harmonic signals, and auto-fluorescence from thesample due to the PARS effect. For example, there may be poor scatteringcontrast, but strong polarization contrast from sample 18. Whileconventional imaging systems may not be configured to find such asignal, polarization-sensitive detection via polarization modulationdetector 23 may provide improved results. By using the additionalinformation contained within the polarization and phase of reflectedbeam 20, added sensitivity may be achieved by averaging across shifts,resulting in lower required optical exposure. Complementary informationmay be found between these shifts which give optical absorptioninformation, and the unshifted values may yield scattering,polarization, and phase in their own right. Such wealth of informationmay be used to drastically improve specificity, since given targets willprovide unique signatures across these six modalities (e.g.,conventional scattering microscope, conventional polarization-basedmicroscope, conventional phase microscope, a PARS microscope, and themicroscopes shown in FIGS. 3A and 3B), allowing for improvedmultiplexing capabilities.

FIG. 4 shows another embodiment of PARS 10 i, in which excitation beam17 and interrogation beam 16 have separated paths, and are not combined.In this embodiment, interrogation beam 16 is focused, using anotherobjective lens 15, to sample 18. In other embodiments, PARS 10 i may besimilar to aspects of both PARS 10 c and 10 d, shown in FIGS. 1C and 2A.Similar to PARS 10 c, PARS 10 i may have multiple excitation lasers,detection lasers, and detection units, the number of which are notparticularly limited.

In some embodiments, both beams may be scanned together. Alternatively,one beam may be fixed while the other beam may be scanned. In otherembodiments, sample 18 may be scanned while both beams are fixed. Sample18 may also be scanned while both beams are scanning. Sample 18 may alsobe scanned while one beam is fixed and the other is scanning.

It will be apparent to one of ordinary skill in the art that other PARSembodiments may be designed with different components to achieve similarresults. For example, other embodiments may include all-fiberarchitectures where circulators replace beam-splitters similar tooptical-coherence tomography architectures. Other alternatives mayinclude various coherence length sources, use of balancedphotodetectors, interrogation-beam modulation, incorporation of opticalamplifiers in the return signal path, etc.

The PARS takes advantage of two focused laser beams on the sample whichmay simulate a confocal PAM configuration.

PARS also takes advantage of optical excitation and detection which mayhelp dramatically reduce the footprint of the system. The absence of abulky ultrasound transducer makes this system suitable for integratingwith other optical imaging systems. Unlike many previous non-contactphotoacoustic imaging systems, the PARS is capable of in vivo imaging.It relies on a much simpler setup and takes advantage of recording thelarge initial ultrasound pressures without appreciable acoustic losses.

During in vivo imaging experiments, no agent or ultrasound couplingmedium are required. However, the target may be prepared with water orany liquid such as oil before a non-contact imaging session. PARS doesnot require a floating table unlike many other interferometric sensors.No special holder or immobilization is required to hold the targetduring imaging sessions. However, a cover slip may be implemented toflatten the target. In some instances, glass windows for the targets,e.g., resected tissue, to sit on may be necessary, and imaging may beperformed through said glass windows. This may help image flat surfacesof the target.

Other advantages that are inherent to the structure will be apparent tothose skilled in the art. The embodiments described herein areillustrative and not intended to limit the scope of the claims, whichare to be interpreted in light of the specification as a whole.

In PARS, a pulse laser is used to generate photoacoustic signals and theacoustic signatures are interrogated using either a long-coherence orshort-coherence length probe beam co-focused with the excitation spots.The PARS may be utilized to remotely record the large local initialpressures from chromophores and without appreciable acoustic losses dueto diffraction, propagation and attenuation.

The excitation beam may be any pulsed or modulated source ofelectromagnetic radiation including lasers or other optical sources. Inone example, a nanosecond-pulsed laser may be used. The excitation beammay be set to any wavelength suitable for taking advantage of optical(or other electromagnetic) absorption of the sample. The source may bemonochromatic or polychromatic.

The interrogation beam may be any pulsed, continuous, or modulatedsource of electromagnetic radiation including lasers or other opticalsources. Any wavelength may be used for interrogation purpose dependingon the application.

The chromatic aberration in the collimating and objective lens pair maybe harnessed to refocus light from a fiber into the object so that eachwavelength is focused at a slightly different depth location. Usingthese wavelengths simultaneously may improve the depth of field and SNRfor structural imaging of microvasculature with OR-PAM.

Since a NI-PARS is not interferometric, the probe/receiver/interrogationbeam of NI-PARS, may be a long-coherence or a short-coherence lengthprobe beam, without need of any reference beam or reference arm. Using ashort-coherence length, however, may ensure preclusion of interferencefrom reflections in the system or sample to avoid unwanted signals andto extract only photoacoustic initial pressures.

Unlike optical coherence tomography (OCT) or interferometry detection ofphotoacoustic signal, the NI-PARS detects the changes in the amount ofthe reflected light from sample due to acoustic pressure and nointerferometry design such as, reference beam, reference arm or axialscanning of reference beam are needed.

Various PARS systems (including, but not limited to PARS, NI-PARS,CG-PARS, C-PARS, and SS-PARS) may be integrated with OCT to provide acomplete set of information offered by both photoacoustic and OCTsystems.

Furthermore, the various PARS with short or long-coherence beams may beused for either optical resolution photoacoustic microscopy (OR-PAM) orcommon photoacoustic microscopy (PAM), or may be combined with 2nd or3rd harmonic, fluorescent, multiphoton, Raman, and/or other,microscopes.

In one example, both excitation and receiver beam may be combined andscanned. In this way, photoacoustic excitations may be sensed in thesame area as they are generated and where they are the largest. Otherarrangements may also be used, including, keeping the receiver beamfixed while scanning the excitation beam or vice versa, and scanning theoptics mechanically while the sample remains stationary, such as, forexample, in a surgical microscope where the patient must remainstationary. Galvanometers, MEMS mirrors and stepper/DC motors may beused as a means of scanning the excitation beam, probe/receiver beam orboth.

The configurations shown in FIGS. 5A-5D may be used to perform PARS andNI-PARS imaging. In the depicted embodiments, excitation beams 502 aredepicted with a larger radius of curvature, and receiver/detection beams504 are depicted with a smaller radius of curvature. FIG. 5A shows anembodiment of a confocal photoacoustic system where excitation beam 502and probing receive beam 504 are focused on the same spot, which can beon a micron- or sub-micron scale. In FIG. 5B, the optical resolution maybe provided by receiver beam 504, rather than excitation beam 502. FIG.5C shows excitation beam 502 and receiver beam 504 focused on differentspots, and takes advantage of ultrasound time of flight in order tolocate excitation beams 502 and receiver beams 504 at differentpositions. In FIG. 5D, optical resolution may be provided by excitationbeam 502. Preferably, the focus of either or both of excitation beam 502and detection beam 504 is less than 30 μm, less than 10 μm, less than 1μm, or to the diffraction limit of light. A tighter focus may result ina higher possible resolution and a better signal to noise ratio in thereflected beam that is detected. As used herein, the term “focus” isintended to refer to the focal zone of the beam, or the point at whichthe beam spot size is at the tightest size, and where the diameter ofthe focal zone is 30% greater than the diameter of the beam spot size.Also preferably, the excitation and detection beams 502 and 504 arefocused on the same position, although there may be some spacing betweenthe respective focuses as shown in FIG. 5C. In FIG. 5C, the beams may befocused at different locations, but preferably within 1 mm, 0.5 mm, 100μm or within the range of the largest focus of the beam. In FIGS. 5A,5B, and 5D, the beams may be confocal, or may overlap within the focusof the beam with the largest focus. For example, in FIG. 5A, excitationbeam 502 is larger than detection beam 504, and detection beam 504 isdirected at a location within the focus of excitation beam 502. Bymoving detection beam 504, the area within excitation beam 502 may beimaged. By having confocal beams, both beams may be moved to image thesample.

One or both of the beams are preferably focused below the surface of thesample. Generally speaking, the beams may be effectively focused up to 8mm (or more) below the surface of the sample. The beams may be focusedat least 50 nm (or even less) below the surface, or focused such thatfocal point of the beam is at least the distance of focal zone of thebeam below the surface of the sample. It will be understood that, whileboth beams are preferably focused below the surface, in some embodimentseither the excitation beam or the interrogation beam may be focusedbelow the surface, with the other focused on, for example, the surfaceof the sample. In cases where only one beam is focused below the surfaceof the sample, the separation between the beams discussed previouslywill be a lateral separation, i.e. in the plane of the sample andorthogonal to the depth of the sample.

The relationship between excitation beams and detection beams,specifically, their focal planes, subsurface of a sample is furtherillustrated in FIGS. 5E-5I. For example, FIG. 5E illustrates a confocalphotoacoustic system including excitation beam 502 and detection beam504, where an excitation focal plane 506 and a detection focal plane 508are focused at the same depth, thereby exhibiting a co-alignmentcondition. This is similarly illustrated in FIG. 5F, except FIG. 5Ffurther illustrates that the co-aligned focal planes 506 and 508 arebelow a glass window 510. Thus, in this instance, co-alignment takesplace through window 510. The distance between glass window 510 andfocal planes 506 and 508 is not particularly limited. FIG. 5G againillustrates co-alignment between focal planes 506 and 508. However, FIG.5G shows that focal planes 506 and 508 are subsurface of sample 512, bya depth defined by a distance 514. Thus, FIG. 5G illustrates excitationbeam 502 and detection beam 504 co-focusing on a spot below the surfaceof sample 512. The depth of focal planes 506 and 508 below the surface512 is not particularly limited, and and in some instances, may rangefrom 100 nm to 1 μm. FIG. 5H illustrates an instance in which excitationbeam 502 is focused, relative to detection beam 504, so that excitationfocal plane 506 is above detection focal plane 508. In contrast, FIG. 5Iillustrates an instance when excitation focal plane 506 is belowdetection focal plane 508. Thus, FIGS. 5H-5I illustrate that focalplanes 506 and 508 may be out of alignment. An example of when focalplanes 506 and 508 are misaligned may be when a PARS system is alignedfor imaging near the surface of a sample, and a user of said PARS systemattempts to focus deeper in the sample without any adjustments. Thisresults in chromatic aberrations, which cause the detection andexcitation focal planes to shift away from one another. Focal planes 506and 508 may be misaligned by 10 μm, 20 μm, 30 μm, etc. However, thedistance between the focal planes is not particularly limited, and maybe any suitable distances. Furthermore, it may be preferable to minimizethe distance between focal planes 506 and 508 for optimal sensitivity.

The excitation beam and detection/receiver beam may be combined usingdichroic mirrors, prisms, beamsplitters, polarizing beamsplitters etc.They may also be focused using different optical paths.

The reflected light may be collected by photodiodes, avalanchephotodiodes, phototubes, photomultipliers, CMOS cameras, CCD cameras(including EM-CCD, intensified-CCDs, back-thinned and cooled CCDs),spectrometers, etc. The detected light may be amplified by an RFamplifier, lock-in amplifier, trans-impedance amplifier, or otheramplifier configuration. Also different methods may be used in order tofilter the excitation beam from the receiver beam before detection. PARSmay use optical amplifiers to amplify detected light.

PARS may be used in many form factors, such as table top, handheld,surgical microscope, and endoscopy. Examples of endoscopy PARS are shownin FIGS. 6A, 6B and 6C with various arrangements of PARS excitationunits 1102, PARS detection units 1104, fibre optics 1106 such asimage-guide fibers, and lenses 1108 that focus the respective beams ontosample 18. When excitation and detection units 1102 and 1104 areseparated, there may be a separate fiber 1110 provided, such as a singlemode fiber.

A table top and handheld PARS may be constructed based on principlesknown in the art. The proposed PARS takes advantage of opticalexcitation and detection which can help to dramatically reduce thefootprint of the system. The footprint of previous systems has been muchtoo large to use the system in all but body surfaces. For endoscopicapplications, the footprint of the ultrasound detector must be minimizedto make the imaging catheter small and flexible enough to navigatethrough small orifices and vessels. The piezoelectric receivers are notideal candidates for endoscopic applications as there is trade-offbetween the sensitivity and the size of the receiver. On the other handfor many invasive applications sterilisable or disposable catheters anda non-contact approach are necessary. The system may also be used asPARS endoscopy system with a potential footprint the size of an opticalfiber, as both excitation and PARS beam can be coupled into a singlemode fiber or image guide fiber.

Image-guide fibers (miniaturized fiber bundles with as many as 100,000or more individual micrometer-sized strands in a single optical fiberwith diameters ranging from 200 μm to 2 mm) may be used to transmit bothfocused light spots. The excitation beam may be scanned either at thedistal end or proximal end of the fiber using one of the scanningmethods mentioned before. However, the receiver beam may be scanned orbe fixed. The scanned spot is transmitted via the image-guide fiber 1106to the output end. Therefore, it may be used to directly contact thesample, or re-focused using an attached miniature GRIN lens 1108. In oneexample, C-scan photoacoustic images were obtained from the fiberimage-guides using an external ultrasound transducer to collectphotoacoustic signals. Using an edge-spread and Gaussian function, aresolution of approximately 7 μm was obtained using the image-guidefiber 1106. It is believed that a higher resolution may also be obtainedwith appropriate improvements to the setup and equipment used. This maybe one possible embodiment of an endoscopic PARS device.

Endoscopic embodiments may also be constructed using single-mode fibersif, for example, the excitation and detection wavelengths aresufficiently close to each other, such as 532 nm and 637 nm. This wouldallow both wavelengths to propagate in single-modes in a highly compactprobe when the fibers are, for example, only 250 microns in diameter.

Endoscopic PARS device embodiments may also be assembled usingdouble-clad fibers. These fibers feature a single-mode core surroundedwith a multi-mode core. This allows for highly dissimilar wavelengths,such as 532 nm and 1310 nm, to be combined into a single fiber whilemaintaining single-mode propagation for at least one of the wavelengths.As well, the double-clad fiber's multimode outer core may be used forincreased return light collection as a means of directing collectedlight towards the optical detection components.

Various PARS embodiments may be combined with other imaging modalitiessuch as fluorescence microscopy, two-photon and confocal fluorescencemicroscopy, Coherent-Anti-Raman-Stokes microscopy, Raman microscopy,Optical coherence tomography, other photoacoustic and ultrasoundsystems, etc. This may permit imaging of the microcirculation, bloodoxygenation parameter imaging, and imaging of other molecularly-specifictargets simultaneously, a potentially important task that is difficultto implement with only fluorescence based microscopy methods. An exampleof this is shown in FIG. 7 , in which a PARS 10 is integrated withanother optical imaging system 1202, where PARS 10 and the other opticalimaging system 1202 are both connected to sample 18 by a combiner 1204.

Interferometric designs, such as common path interferometer (usingspecially designed interferometer objective lenses), Michelsoninterferometer, Fizeau interferometer, Ramsey interferometer, Sagnacinterferometer, Fabry-Perot interferometer and Mach-Zehnderinterferometer, may also be integrated with various embodiments of thedisclosure.

A multi-wavelength visible laser source may also be implemented togenerate photoacoustic signals for functional or structural imaging.

Polarization analyzers may be used to decompose detected light intorespective polarization states. The light detected in each polarizationstate may provide information about ultrasound-tissue interaction.

Applications

It will be understood that the system described herein may be used invarious ways, such as those purposes described in the prior art, andalso may be used in other ways to take advantage of the aspectsdescribed above. A non-exhaustive list of applications is discussedbelow.

The system may be used for imaging angiogenesis for differentpre-clinical tumor models.

The system may be used to image: (1) histological samples; (2) cellnuclei; (3) proteins; (4) cytochromes; (5) DNA; (6) RNA; and (7) lipids.

The system may also be used for clinical imaging of micro- andmacro-circulation and pigmented cells, which may find use forapplications such as in (1) the eye, potentially augmenting or replacingfluorescein angiography; (2) imaging dermatological lesions includingmelanoma, basal cell carcinoma, hemangioma, psoriasis, eczema,dermatitis, imaging Mohs surgery, imaging to verify tumor marginresections; (3) peripheral vascular disease; (4) diabetic and pressureulcers; (5) burn imaging; (6) plastic surgery and microsurgery; (7)imaging of circulating tumor cells, especially melanoma cells; (8)imaging lymph node angiogenesis; (9) imaging response to photodynamictherapies including those with vascular ablative mechanisms; (10)imaging response to chemotherapeutics including anti-angiogenic drugs;(11) imaging response to radiotherapy.

The system may be useful in estimating oxygen saturation usingmulti-wavelength photoacoustic excitation and PARS detection andapplications including: (1) estimating venous oxygen saturation wherepulse oximetry cannot be used including estimating cerebrovenous oxygensaturation and central venous oxygen saturation. This could potentiallyreplace catheterization procedures which can be risky, especially insmall children and infants.

Oxygen flux and oxygen consumption may also be estimated by using PARSimaging to estimate oxygen saturation, and an auxiliary method toestimate blood flow in vessels flowing into and out of a region oftissue.

The system may also have some gastroenterological applications, such asimaging vascular beds and depth of invasion in Barrett's esophagus andcolorectal cancers. Depth of invasion is key to prognosis and metabolicpotential. Gastroenterological applications may be combined orpiggy-backed off of a clinical endoscope and the miniaturized PARS maybe designed either as a standalone endoscope or fit within the accessorychannel of a clinical endoscope.

The system may have some surgical applications, such as functionalimaging during brain surgery, use for assessment of internal bleedingand cauterization verification, imaging perfusion sufficiency of organsand organ transplants, imaging angiogenesis around islet transplants,imaging of skin-grafts, imaging of tissue scaffolds and biomaterials toevaluate vascularization and immune rejection, imaging to aidmicrosurgery, guidance to avoid cutting critical blood vessels andnerves.

Other examples of applications may include PARS imaging of contrastagents in clinical or pre-clinical applications; identification ofsentinel lymph nodes; non- or minimally-invasive identification oftumors in lymph nodes; imaging of genetically-encoded reporters such astyrosinase, chromoproteins, fluorescent proteins for pre-clinical orclinical molecular imaging applications; imaging actively or passivelytargeted optically absorbing nanoparticles for molecular imaging; andimaging of blood clots and potentially staging the age of the clots.

In this patent document, the word “comprising” is used in itsnon-limiting sense to mean that items following the word are included,but items not specifically mentioned are not excluded. A reference to anelement by the indefinite article “a” does not exclude the possibilitythat more than one of the elements is present, unless the contextclearly requires that there be one and only one of the elements.

The scope of the following claims should not be limited by the preferredembodiments set forth in the examples above and in the drawings, butshould be given the broadest interpretation consistent with thedescription as a whole.

1. A photoacoustic remote sensing system (PARS) for imaging a subsurfacestructure in a sample, comprising: one or more laser sources configuredto generate a plurality of excitation beams configured to generatepressure signals in the sample at an excitation location; wherein theone or more laser sources are also configured to generate a plurality ofinterrogation beams incident on the sample at the excitation location, aportion of the plurality of interrogation beams returning from thesample that is indicative of the generated pressure signals; an opticalsystem configured to focus the plurality of excitation beams at a firstfocal point and the plurality of interrogation beams at a second focalpoint, the first and second focal points being below the surface of thesample; and a plurality of detectors each configured to detect areturning portion of at least one of the plurality of interrogationbeams. 2.-29. (canceled)